The present invention relates generally to an ion source capable of providing an ion beam and especially a high-brightness, heavy current ion beam.
Generally speaking, within conventional ion sources which utilize electron bombardment ionization due to gaseous discharge, the correlation among normalized emittance E representative of the propriety of the path of ions, normalized brightness B and ion beam current I can be given below: ##EQU1## WHEREIN C is the proportionality constant, e is the charge of electrons, N is the plasma density, Te is the electron temperature and Ti is the ion temperature.
Now assume that the following proportional relation is viewed between the electron temperature Te and the ion temperature Ti. ##EQU2## WHEREIN C' is the proportionality constant, m is the mass of electrons and M is the mass of ions.
If the plasma density N is rendered constant in dependence upon the type of ion sources, then the normalized brightness B and the normalized emittance E will be rewritten: ##EQU3## EQU E .varies. I
The dependencies of the brightness B and the emittance E as defined above are experimentally ascertained within ion sources of the high frequency discharge type, the P.I.G. type, the electron bombardment type, the duoplasmatron type, etc. However, if the plasma density N can be increased, then the different relation B.varies.I will exist because the plasma density N varies in proportion to the current I. The brightness B will be therefore increased with increase in the current I. As a matter of fact, heavy current can be permitted to flow without decreasing at least the brightness even though the brightness B can not be increased in proportion to the current due to any other parameters.
Accordingly, it is an object of the present invention to provide an ion source capable of producing a high brightness, heavy current ion beam, taking account of the above discussed aspects.